1. Field of the Invention
This invention provides a bladed anemometer which has the capability of accurately sensing fluid flow under varying environmental conditions and varying angles of fluid attack. This invention, for the first time, permits sensing of fluid movements in hostile environmental conditions such as very high or very low temperature and high dust and moisture concentrations.
2. Prior Art
Anemometers utilizing a helicoidal blade design have been used for many years. This blade design was employed in air speed indicators of early aircraft because it had a superior response to side vectors. This design has also been used in previous fixed-blade anemometers such as the one built by Gill, et al. However, this instrument has shown an error, the magnitude of which varied with varying angles of fluid attack. The angle of attack is the angle between the anemometer axis and the fluid flow direction. Thus fluid flowing parallel to the direction of anemometer rotation would be termed to have an angle of attack of 90.degree. while fluid flowing parallel to the shaft about which the blades rotate would have a 0.degree. angle of attack. Additionally, the number of blades has been found to be a major factor affecting accuracy. In typical four-bladed anemometers, the error is approximately 10-20% at an angle of attack of 60.degree.. For a three-bladed anemometer at an angle of attack of 80.degree., the error can be as much as 200%. With a given angle of attack, the total error has been found to depend on the number of blades and to a lesser extent the fluid speed.
The error associated with present anemometers has been attributed to the von Karman vortices shed by the shaft on which the anemometer is mounted. To counter these forces, various techniques such as roughening of the shaft and shaft extenders have been tried with little success. The failure of these corrective measures is due to the fact that these techniques did not attack the real source of the problem. Our research has shown that the sensing error of present anemometers results from vortices shed by the ends and edges of the anemometer blades as they move through the fluid.
Each blade of an anemometer acts as a miniature wing and, similarly to the wing of an aircraft, it has a tip vortex associated with it just as does an aircraft wing when producing lift forces. These lift forces become more important as the angle of fluid flow relative to the axis of the anemometer increases, and these forces vary in strength as the relative angle of attack of the blades changes during rotation. Each of the vortices interacts with all others in a complex manner but the general effect is to cause the vortices to rotate about each other with the axis of this vortex rotation spiraling away from the blades. Because of the interaction between the vortices, some of the vortices can be displaced so as to generate a change in the dynamic pressure on neighboring blades, and this pressure can be strong enough in some instances to reverse the direction of rotation or cause an increase in the speed of rotation.
No previous anemometer designs have been capable of continued operation in hostile environments such as where temperature, moisture or dust extremes exist. Thus in situations such as forest fires, remote installations at very high altitudes where low temperatures are reached or in areas where temperature extremes of both hot and cold were encountered, a practical method for continuously or regularly monitoring wind flow was nonexistent. The data derived from a forest fire situation would be useful in predicting the future course of the fire and planning countermeasures. In high altitude situations, such data might give important information as to the location and extent of snow accumulations thus facilitating road construction and maintenance planning. These are merely examples of situations in which accurate data on wind flow would be extremely valuable but which cannot be readily measured with existing equipment.
Thus one of the purposes of this invention is to eliminate the errors that have been associated with measurement of fluid velocity in previous devices.
Another purpose of this invention is to provide a means for measuring fluid velocity in rapidly varying, hostile environments.
A further purpose of this invention is to provide a device, the data from which may be easily and readily recorded, retrieved and analyzed.